I41/acd is number 142 of the 230 space groups: a body-centred tetragonal lattice carrying the point-group class 4/mmm. A general position in its cell is repeated 32 times. It is written I 41/a 2/c 2/d in full, which spells out the direction of every element.
It holds an improper operation, so a single enantiomer cannot crystallise in it. It contains the inversion, which halves the number of independent reflections and settles the phase problem differently from a non-centrosymmetric group. It is not symmorphic: no choice of origin removes its glide planes and screw axes, so systematic absences give it away in the diffraction pattern.
A diffraction pattern from a crystal in it shows the Laue symmetry 4/mmm, so that is the most the symmetry of the intensities can tell you before the structure is solved.
Molecular point groups with their operations and character tables, the 230 space groups with a cell builder, and the 17 wallpaper and 7 frieze groups drawn live. The tool needs JavaScript, and WebGL2 for the 3D views; these are the pages it offers: